.box {
    position: relative;
    margin: 30px;
    width: 450px;
    display: inline-block;
}

.img {
    width: 400px;
    height: 400px;
    border: 1px solid #333;
    position: relative;
    background: url(./imgs/1.jpg) no-repeat;
}

.img>.mask {
    width: 200px;
    height: 200px;
    background-color: yellow;
    opacity: .4;
    position: absolute;
    top: 0px;
    left: 0px;
    display: none;
}

ul {
    display: flex;
    list-style: none;
    margin-top: 10px;
}

ul>li {
    margin-right: 10px;
    padding: 10px;
    cursor: pointer;
}


.enlarge {
    width: 200px;
    height: 200px;
    position: absolute;
    top: 0;
    left: 100%;
    background: url(./imgs/1.big.jpg) no-repeat;
    background-size: 200px 200px;
    background-position: 0 0;
    display: none;
}